1. Field of the Invention
The field of the invention is that of digital data transmission systems.
More specifically, the invention relates to transmission systems that enable the conveying of a specific message by RF links to a given receiver or solely to this receiver.
It can be applied notably to systems in which at least certain receivers work intermittently (i.e. go into operation periodically, at predetermined instants).
A favored field of application of the invention is that of unilateral paging. The term paging refers to any system that enables the transmission of a message to a portable pager or receiver that can advantageously be contained in the user's pocket. These systems can notably be used to transmit a message to a person who is away from his office when the sender of the message is unaware of his whereabouts.
2. Description of the Prior Art
In a standard way, and in the most simple case, such systems provide for the emission of a sound and/or visual signal by the pocket receiver when a message is received. These systems are generally known as simple person call systems. When the subscriber who has the receiver receives a message, he makes a phone call to a pre-determined number, for example that of his secretary's office, or interrogates a voice message system (by telephone) or computer message system, by means of a teletext terminal such as a Minitel (registered mark) unit.
Other improved receivers enable the reception and display of a short digital message, for example a telephone number to be called or a coded information element.
Receivers such as these are notably used in radio paging systems known by the names "Operator" (registered mark) and "Alphapage" (registered mark) available in France.
These radio paging systems, just like all the other presently known radio paging systems, have many drawbacks.
Firstly of all, they enable the transmission, at best, of only a short message. This means that in practice, the subscriber receiving a message must routinely look for a telephone to find out the contents of the information intended for him. This is a cumbersome procedure that is sometimes difficult to put into practice.
It may be imagined, for example, that it is difficult to find a telephone and even more difficult to find a Minitel (registered mark) on a motorway. In this case, the time at which the user gets to know the information will be greatly delayed, and this may sometimes have detrimental effects.
Furthermore, since portable receivers can be powered only by cells or batteries, these receivers must work intermittently for obvious reasons of operating autonomy. More specifically, a radio paging receiver is most usually in a stand-by watch state (only a clock is powered) and goes into cyclical operation to detect a message, if any, that is intended for it.
This implies a particular system of management of the instants of transmission, so that each message is sent only when the intended receiver has left the stand-by watch state to go into operation.
On the other hand, the receivers must have specific synchronization systems so as to control the instants at which they go into operation. To achieve this, in the known systems, temporal synchronization data elements are inserted among the radio paging messages. For example, in the case of the Operator (registered mark) system, codes representing minutes and tenths of minutes are sent. A major drawback of this technique is that it is necessary to decode all the information elements transmitted up to the detection of a synchronization code, which keeps the receiver in operation for a certain period of time, which means that electrical power is consumed.
The patent application No. WO-8 603 645 also presents a technique for the synchronization of a radio paging system based on a division of time. More specifically, this document proposes the division of time into intervals (known as frames, sub-frames and time slots), the numbering of these slots and the assigning of the numbers to the receivers.
Again, the receivers are required to have precise knowledge of the time in order to get activated at the right instants. To do this, the system carries out the transmission, in each sub-frame, of an information element bearing the date and time. At each activation, the receiver must therefore read this information element and reset its clock accordingly, if necessary. These operations also keep the receiver in operation for a certain period of time, leading to high power consumption.
Furthermore, this radio paging system, as well as many present systems of radio paging, require a complete specific structure and notably particular transmitters. This is also the case with the Alphapage (registered mark) system.
In this case, the costs of installing a radio paging system of this type are high and are reflected, firstly, in the cost of the receivers and, secondly, in the cost of the communications.
Furthermore, the need for these specific installations greatly restricts the geographical coverage of these radio paging systems, only bigger zones (bigger in terms of traffic) being covered in general.
In other cases, the radio paging systems rely on an already existing frequency modulation broadcasting network. The infrastructural costs are then lower, but this is achieved at the cost of greater complexity of the receivers. In particular, since the geographical coverage of a transmitter working by frequency modulation is limited, it is necessary, if the radio paging service is to cover a huge territory, for the each receiver to include demodulation frequency changing means and means for the selection of the best demodulation frequency at each instant (RDS or Radio Data System).
Furthermore, known broadcasting systems meet with difficulties of FM signal reception in many situations, especially when they are mobile and/or are used in urban environments (due to the Doppler effect, fading, echos, multiple paths, jamming etc.). They therefore do not offer sufficient guarantees of reception.
It is notably an object of the invention to overcome these different drawbacks of the prior art.